For example, a Group III nitride compound semiconductor used in a light-emitting device is a direct transition type semiconductor exhibiting an emission spectrum in a wide range of from ultraviolet to red. The Group III nitride compound semiconductor is applied to a light-emitting device such as a light-emitting diode (LED), a laser diode (LD), or the like. Application of the Group III nitride compound semiconductor to a transistor such as an FET or the like has been developed actively because the band gap of the Group III nitride compound semiconductor is so wide that the device using the Group III nitride compound semiconductor can be expected to operate more stably at a high temperature than a device using another semiconductor. Development of the Group III nitride compound semiconductor into various general semiconductor devices has been expected from the environmental viewpoint because arsenic is not used as the main component. Generally, the Group III nitride compound semiconductor is formed on sapphire used as a substrate.
If the Group III nitride compound semiconductor is formed on such a sapphire substrate, dislocation is caused by misfit in lattice constant between sapphire and the Group III nitride compound semiconductor. For this reason, there is a problem that device characteristic becomes poor. The dislocation caused by the misfit is threading dislocation that passes through a semiconductor layer vertically (in a direction perpendicular to a substrate surface). There is a problem that dislocation of about 109 cm−2 propagates in the Group III nitride compound semiconductor. When Group III nitride compound semiconductor layers different in composition are laminated, it propagates up to the uppermost layer. For example, in the case of a light-emitting device, there is therefore a problem that device characteristic such as threshold current in LD, device life in LD and LED, etc. cannot be improved. Also in the case of another semiconductor device, no device but a semiconductor device low in mobility can be formed because electrons are scattered by defects. These problems occur also in the case where another substrate is used.
For example, there has been accordingly proposed a method in which: a Group III nitride compound semiconductor layer is once formed on a substrate with or without interposition of a buffer layer; a mask is formed on part of an upper surface of the Group III nitride compound semiconductor layer so that the Group III nitride compound semiconductor cannot be epitaxially grown vertically from the masked portion; and the Group III nitride compound semiconductor is epitaxially grown vertically and laterally from the non-masked portion to thereby form a Group III nitride compound semiconductor layer low in threading dislocation above the masked portion.
In the technique described in the official gazette, it is however necessary to perform Group III nitride compound semiconductor growth twice before and after masking because different apparatuses are used for Group III nitride compound semiconductor growth and masking respectively. Moreover, in the technique described in the official gazette, a space is apt to be formed between the Group III nitride compound semiconductor layer formed above the masked portion and the mask because the Group III nitride compound semiconductor cannot be formed on them asked portion with good adhesion. As a result, when separation into devices is performed in an after-process, peeling, cracking or chipping occurs in the portion where the space is formed.
The invention has been made to solve the aforementioned problems and an object of the invention is to provide a semiconductor device in which a Group III nitride compound semiconductor layer suppressed in terms of threading dislocation is formed by a smaller number of steps as a whole and in which peeling, cracking or chipping hardly occurs.